JSDT looks at all functions in the given source folder and can list them in content-assist. Taking advantage of this, we use a dummy definitions file called apis.sah which has all the Sahi APIs. This file, apis.sah, can be downloaded from here and needs to copied into the "scripts" folder.
JSDT is a part of the Web Platforms Toolkit.
It can be installed as a plugin or can be downloaded as a single bundle in the "Eclipse IDE for Java EE Developers (163 MB)" (For windows: eclipse-jee-ganymede-SR2-win32.zip). This is downloadable from http://www.eclipse.org/downloads/